Weather modification in Alberta : summary report and recommendations

Research and operations

1980-1985

Weather modification

in Alberta

Summary report and recommendations

ALBERTA

RESEARCH

COUNCIL

Natural Resources Division

Atmospheric Sciences Department

Digitized by the Internet Archive
in 2016

https://archive.org/details/weathermodificatOOalbe

™NADrAMf

Research and operations

1980-1985

Weather modification

in Alberta

Summary report and recommendations

October 1986

Cover photograph left to right

A severe hailstorm that occurred in Alberta on August 1 1 ,
1972.

Radar systems at the Red Deer Airport used for weather
research (large antenna), cloud seeding control (dome-
covered radar), and aircraft tracking (flat antenna).

Research aircraft used for studying hailstorms. On the left is
the Cessna Conquest turboprop aircraft developed by IN-
TERA and the Research Council. This aircraft, equipped with
state-of-the-art laser sensors, measures properties of clouds
before and after seeding. These are recorded and analyzed
by an onboard computer system. On the right is an armour-
plated T-28 aircraft from the South Dakota School of Mines
and Technology. This aircraft measures conditions in the
heart of the storm.

A promising wheat crop near Delburne, destroyed by hail on
August 11, 1982.

ALBERTA

RESEARCH

COUNCIL

October 1, 1986

Honorable Peter Elzinga, Minister of Agriculture
Honorable Shirley Cripps, Associate Minister of Agriculture
Legislature Building
Edmonton, Alberta
T5K 2B6

Dear Ministers:

I have the honor to transmit, herewith, the Summary Report and Recommendations of the Alberta Research
Council on the Research and Operations of the 1980 to 1985 Weather Modification Program. A forthcoming
report, comprising technical papers, will provide the detailed scientific results of the program from which this
report is derived.

Respectfully yours,

R.W. Stewart
President

Alberta Research Council

250 Karl Clark Road
Edmonton, Alberta
Canada
403/450-5111

Telex 037-2147
(RESEARCH EDM)

Mailing address:

PO Box 8330
Postal Station F
Edmonton, Alberta
Canada T6H 5X2

Preface

i

The Alberta Research Council conducted a research
and operational weather modification program in cen-
tral Alberta during the period 1980-1985 inclusive. The
year 1980 was a transition year in which the Alberta
Research Council assumed management of the
weather modification project. The current five-year ex-
perimental program started in 1981. Hail suppression
through cloud seeding has been the focal point of this
program, with exploratory studies of cloud seeding to
increase rain and snow.

The cost of the five-year program was $22,730,000
—of which $17,420,000 was contributed by Alberta
Agriculture and $5,320,000 by the Alberta Research
Council.

This document constitutes the summary report of,
and recommendations for, the program. It contains a
brief history of weather modification activities in Alber-
ta, the rationale for, and objectives of, the 1 980 to 1 985
program and the conclusions and recommendations.
A brief summary of applications of technology
developed for weather modification research to non-
weather-modification problems is also included.

The research results are presented in greater detail
in an accompanying report, which consists primarily of
technical papers. Analysis of the data obtained during
the last five years is far from complete. New results will
be forthcoming within the next year which will lead to
more definitive conclusions.

Acknowledgments

This past five year weather modification research pro-
gram was a major undertaking that has gained enor-
mously by the participation of many people and or-
ganizations. The inspiration and support of Drs. G.
Goyer and B. Barge in the program’s early stages are
acknowledged. The Alberta Research Council has
benefited from and appreciates the sage advice, pa-
tience and encouragement provided by the Advisory
Committee on Weather Modification under the chair-
manship of Mr. J. Christie. The financial support of
Alberta Agriculture is most gratefully acknowledged
with special recognition to Dr. A. Olson for his effective
liaison between Alberta Agriculture and the Alberta
Research Council.

The interaction with our colleagues at the Atmos-
pheric Environment Service, and various universities
and research agencies in Canada and around the
world is acknowledged and appreciated. In particular,
the guidance provided by Dr. P. Smith of the South
Dakota School of Mines and Technology, during his
sabbatical at the Alberta Research Council was most
helpful.

The Technical Review Committee of Chairman Dr.
R.F. Reinking, U.S. Dept, of Commerce, Mr. S.A.
Changnon, Illinois State Water Survey, Dr. H-R. Cho,
University of Toronto, Dr. J.A. Flueck, University of
Colorado, Dr. G.B. Foote, U.S. National Center for At-
mospheric Research, Mr. T.J. Henderson, Atmos-
pherics Inc. Fresno, CA and Dr. G.A. Isaac, Atmos-
pheric Environment Service, Toronto, who met at the
Research Council in late 1985, are acknowledged with
thanks for their many helpful suggestions concerning
the analysis of the research results and ideas re-
garding future weather modification research.

The valuable economic study referred to in this
report, carried out by Dr. C. Ross and by P. Woloshyn
of the Economic Services Division of Alberta Agri-
culture, is also acknowledged.

Lastly, a most sincere acknowledgment to the
farmers of Alberta who contributed so greatly to the
success of the program with their unceasing support,
particularly in taking rain and hail measurements and
answering untold numbers of telephone inquiries
regarding hailstorms and crop damage.

The average annual loss-to-risk ratio by township based on records of the Alberta Hail and Crop Insurance Corporation. The
red and black squares indicate townships with average losses exceeding 9 and 12% respectively. Blank spaces indicate areas
with less than 20 years of insurance data.

Executive summary

II!

The importance of agriculture to the Alberta economy
and the heavy costs associated with hail damage and
drought have focused attention on the use of weather
modification techniques. Because agriculture con-
tributes $3 billion to the provincial economy, adverse
weather can be devastating. For example, during the
period 1980 to 1985, average annual hail damage cost
about $150 million. In addition, the 1985 drought cost
farmers an estimated $650 million. The most common
type of weather modification is cloud seeding, which
involves injecting clouds with dry ice or silver iodide.
Results of the last five years of research in the pro-
vince lead to optimism about the use of cloud seeding
to decrease hail and to increase rain and snow.

Research results

A better understanding of the physical processes that
produce hail has been gained by investigating the
whole precipitation process right from the large-scale
synoptic weather patterns associated with fronts and
low pressure systems to the microscale precipitation
processes that occur within a cloud. The current state
of knowledge about the Alberta hailstorm has been
demonstrated to be essentially correct.

Results from hailstorm seeding experiments in-
dicate that cloud seeding can increase the number of
potential hail embryos or (in some storms) can cause
potential hail embryos to precipitate prematurely.
Evidence has been found that there are more hail-
stones on the ground from operationally seeded
storms, but more observations would be required to
determine if the size of the hailstones is significantly
changed.

Because of limitations in the measuring and observ-
ing facilities, research has not yet demonstrated that
more hail embryos lead to smaller hailstones or that
fewer embryos result in less hail on the ground.

Computer calculations suggest that cloud seeding
decreases hail (by nine percent) and increases rain (by
seven percent). Research based on analysis of crop
damage suggests a decrease in the loss-to-risk ratio of
about 20 percent. However, this decrease, or parts of
it, could be due to other factors such as climate
change or insurance practices rather than to cloud
seeding.

These results give reason to be optimistic about sup-
pressing hail in the most common types of storms.

Research also indicates that towering cumulus
clouds can be made to rain by seeding, even when
such clouds would not rain naturally. This is significant
since towering cumulus clouds are just as prevalent in
drought periods as they are in periods of normal rain-
fall.

Limited observations of snow clouds over the Rocky
Mountains in southern Alberta imply that a potential
exists for increasing mountain snowpack.

Based on a 20-year operating period, the capital and
operating costs of a cloud seeding program in Alberta
would be recovered if annual crop losses were re-
duced by about five percent. Given experimental
evidence to date, this is a realistic goal. Rain and hail

experiments could be easily combined, as aircraft,
radar, weather office assistance and other observa-
tional support are the same for both.

Recommendations

Of the three forms of weather modification considered,
that is hail suppression, rain augmentation and snow
increase, the hail problem is the most complex and dif-
ficult to solve, but substantial progress has been made
and hail suppression looks feasible at least for some
types of storms. It is recommended that hail suppres-
sion research receive continued support to address
the questions that remain. It is also recommended that
continued analysis of the data obtained from the 1980
to 1985 program be supported to provide more
definitive conclusions about the effectiveness of
weather modification and to help focus future research
efforts. Further experimentation is required and
recommended to optimize the seeding technology. In
particular, seeding delivery techniques should be ex-
amined to improve coverage of storms.

Alberta support is recommended for an international
project called Hailswath II. Participants from the
United States and Europe would gather in Alberta in
1988, bringing their equipment, to carry out an inten-
sive hailstorm study to address some of the questions
that still remain.

Due to the success of the rain augmentation project,
it is recommended that the rain research program be
expanded.

Funding is also recommended for the SNOWATER
project, proposed to adapt snow augmentation tech-
nology for use in Alberta.

Operational cloud seeding should be carried out by
the private sector. However, every effort should be
made to evaluate its effectiveness and a strong link
should be maintained between the research and
operational components of future weather modification
programs.

Additional benefits

Weather modification research results in the develop-
ment of a broad range of expertise and supporting
technology which can be applied to other areas of
government and private enterprise. The meteorologi-
cal expertise associated with the weather modification
evaluations has been applied to crop yield modelling
for Alberta Energy and Natural Resources and in oil
production studies for the Alberta Oil Sands Tech-
nology and Research Authority (AOSTRA). Some of
the computer programs have been used by Syncrude
in environmental work.

A research aircraft developed jointly by INTERA
Technologies and the Alberta Research Council has
been used in air pollution studies for Alberta Environ-
ment in addition to weather modification research. IN-
TERA has also been successfully marketing the
technology and experience gained from its involve-
ment with the weather modification project in the inter-
national marketplace. They now have a three-year, $10

iv

million weather modification contract with the Greek
government. Data collected by the aircraft during the
weather modification program will be used by
American researchers in a study aimed at revising ic-
ing standards for commercial aircraft.

The radar data collected as part of the weather
modification program have been analyzed for Alberta
Environment to assist in assessment of severe weather
events and applied to the development of forecasts of
damaging lightning for the Canadian Electrical Associ-
ation. Radar data are also being used by Alberta
Government Telephones to study signal fading during
heavy rainfall. These contracts indicate that other
Alberta agencies are developing a reliance on the
weather radar system that supports weather modifica-

tion research.

These examples demonstrate how the offshoots of
research into weather modification help keep Alberta
in the forefront of technology in many related fields.

Conclusion

The potential economic gains to agriculture from cloud
seeding are substantial. Furthermore, the Alberta
Research Council has developed a world-class team of
scientists, and advanced technologies with potential
applications in many areas of industry. Therefore, it is
recommended that weather modification receive con-
tinued support.

Contents

v

Preface i

Acknowledgments i

Executive summary iii

Research results iii

Recommendations iii

Additional benefits iii

Conclusion iv

Introduction 1

The potential of weather modification 1

The history of weather modification in Alberta 1

Objectives for the period 1 980-1985 2

Operational cloud seeding 2

Cloud seeding techniques 2

Cloud-top seeding 2

Cloud-base seeding 4

Hail

Alberta hailstorms 4

The growth of a hailstone 5

Hail suppression concepts 6

Testing the concepts 7

Experimental results 8

Hailstorm seeding experiments 8

Computer calculations 11

Economic assessment of hail suppression techniques 11

Summary and conclusions 11

Rain

The potential of rainfall augmentation 12

Rainfall augmentation concepts 12

Results of the seeding experiments 13

Cloud climatology studies 13

Alternative seeding techniques 13

Cloud-base seeding 13

Ground-based seeding 13

Summary and conclusions 15

Snow

Potential of snowpack augmentation .15

Snowfall augmentation concept 15

Alberta investigations 15

Summary and conclusions 16

Technological benefits of the program 16

Recommendations

Hail 17

Rain 17

Snow 17

Technology development 18

Conclusion 18

Introduction

i

The potential of weather
modification

The success of Alberta’s $3 billion agricultural industry
depends largely on the weather. Drought, hail, frost,
floods and tornadoes cost Albertans hundreds of
millions of dollars annually.

During the period 1980 to 1985, hailstorms caused
an average $150 million annually in direct losses to
crops. Secondary economic losses could add another
$50 million to this amount. On the rare occasions when
a severe hailstorm passes over a large city, the losses
can be even more dramatic. A hailstorm that hit
Calgary on July 28, 1981, caused an estimated $125
million damage and a hailstorm that passed over
Munich, Germany on July 12, 1984 is thought to have
caused one billion dollars damage.

Lost agricultural production due to insufficient
moisture is also estimated at $150 million per year.
The severe drought of 1985 is considered to have cost
Alberta farmers $650 million.

The devastating effects of hail and the lack of ade-
quate water supplies have focused attention on the
use of weather modification techniques to alleviate the
problem. Weather modification potentially has a very
favorable cost-benefit ratio. A five percent reduction in
annual crop losses would be required to recover the
capital and operating costs of a cloud seeding pro-
gram.

The history of weather modification
in Alberta

Crop losses due to hail damage in the early 1950s
prompted farmers in the area north and east of Calgary
to form the Alberta Weather Modification Co-operative

and hire I.P. Krick and Associates to carry out a com-
mercial hail suppression program.

In 1956, a project was initiated by the Alberta
Research Council to study Alberta hailstorms in order
to design and test means for suppressing hail. Par-
ticipants in this early project included the Atmospheric
Environment Service of Environment Canada, the Na-
tional Research Council and the Stormy Weather
Group at McGill University. This phase of research
continued until 1968. During these early years, fun-
damental information was collected on the
characteristics of hail in Alberta such as its frequency
at particular locations, the size of hailstones, the size
and duration of hailstorms, and the weather patterns
associated with hailstorms.

By 1969, it was believed the understanding of Alber-
ta hailstorms was sufficient to begin a series of air-
borne cloud seeding experiments called Project
Hailstop. By 1973, a seeding technology had been
developed whereby silver iodide flares were dropped
into the tops of clouds developing on the edges of
hailstorms.

The provincial government created the Alberta
Weather Modification Board (under the Department of
Agriculture) in 1973 to direct weather modification
research and operations. The major emphasis of the
research was a randomized seeding test. Using obser-
vations obtained by interviewing farmers about the
time, location and nature of hailfall, and those obtained
from weather radar, a statistical comparison of data
from seeded and nonseeded storm days was con-
ducted. Results were inconclusive because none of
the various measurements of hailfall were accurate
enough to reliably detect a seeding effect over the
short time period of the project.

In 1980, after an interim year, the Alberta Research

Figure 1. Estimated total annual crop damage in Alberta due to hail (expressed in 1985 dollars).

2

Council assumed full responsibility for research and
operations in weather modification. An Advisory Com-
mittee on Weather Modification was formed by Alberta
Agriculture to provide the Alberta Research Council
with guidance on the general direction of the program
from the point of view of the farm community. Up to
this point the program had dealt exclusively with hail
suppression, but it was now expanded to include rain
and snow increase studies.

Objectives for the period 1980-1985

During the period 1980 to 1985, the weather modifica-
tion program emphasized a better understanding of
the physical processes occurring in storms and the ef-
fects of cloud seeding upon them. This seemed essen-
tial to evaluate seeding effects reliably in a relatively
short time period and to develop optimum seeding
techniques. The overall goal of the program was to
gain enough information to assess the feasibility of
conducting, at a later date, a definitive cost-benefit
analysis of weather modification in Alberta. Although
the primary emphasis was on hail suppression, secon-
dary objectives were rain increase and snowpack aug-
mentation.

To understand the physical processes that produce
hail, it is necessary to investigate the whole precipita-
tion process right from the large-scale synoptic
weather patterns associated with fronts and low pres-
sure systems to the microscale precipitation processes
that occur within a cloud. Hailstorms vary considerably
in intensity, size and duration. These variations are
now known to be determined by the environment in
which the storm occurs and, for the most part, by pro-
cesses that begin several hours before the storm
forms. This investigation required a considerable
amount of advanced technology.

The overall goal of the program incorporated
five specific objectives:

1. To assess whether cloud seeding sufficiently in-
fluences hailstorms to cause a substantial change
in hailfall. To meet this objective an aircraft equip-
ped with cloud physics instrumentation, a weather
radar facility and an upper air observing system
were used. The aircraft facility was developed jointly
by INTERA Technologies and the Alberta Research
Council. In preparation for a future economic evalu-
ation of cloud seeding for hail suppression, fore-
casting techniques were improved to provide
methods to predict storm behavior, help detect
seeding effects and improve cloud seeding opera-
tions. New observational techniques were investi-
gated to provide measurements of hailfall that could
be used to evaluate cloud seeding.

2. To assess the potential for increasing rainfall by
seeding cumulus clouds. For this, the same aircraft
and radar facilities were used.

3. To investigate the potential for increasing snowfall
in the Alberta Rocky Mountains by cloud seeding.
The aircraft facility was also used for this.

4. To investigate optimum seeding techniques for
various types of weather modification, using two

seeding materials (silver iodide and dry ice); as well
as three delivery methods (seeding at cloud top;
seeding at cloud base with aircraft; and seeding
with ground-based generators). In particular, the
ability of ground-based generators to inject suitable
amounts of seeding material into clouds was
evaluated.

5. To routinely seed hailstorms using aircraft.

Operational cloud seeding

During the program from 1980 to 1985, hailstorms
were routinely seeded by aircraft from June 20 to
August 31 in the region from Calgary to Red Deer, car-
rying on the earlier seeding program of the Alberta
Weather Modification Board. Seeding was also con-
ducted north of Red Deer until 1981. Seeding aircraft
and operations staff were under contract to INTERA
Technologies of Calgary. Six seeding aircraft were
used in 1980 and 1981 with five from 1982 to 1985, in-
cluding a special “research seeder” for the rain and
hail seeding experiments. Each aircraft was equipped
with flares and acetone solution generators for
seeding. Aircraft were directed to the storms from the
radar control site at the Red Deer airport.

Cloud seeding techniques

Since the discovery of silver iodide as an ice forming
(nucleating) material in 1946, a number of cloud
seeding techniques have been developed to generate
the seeding material and dispense it into the clouds.
Usually, the microscopic particles are generated by
burning silver iodide rods in an electric arc, burning
silver iodide and acetone solution or by burning silver
iodide in a pyrotechnic mixture in flares or rockets.
Rockets containing a silver iodide charge are used ex-
tensively in the Soviet Union, and a number of its
satellite countries, and in China for hail suppression
seeding. Most other countries are using, or have used,
ground-based generators or flares carried by aircraft to
deliver seeding materials to clouds.

Cloud-top seeding

Cloud-top seeding involves the discharge of small
pyrotechnic flares, carried under the belly of the air-
craft, into the tops of developing cloud towers (feeder
clouds). Flares are dropped one km apart as the
seeding aircraft flies over the cloud towers. The aircraft
returns within a few minutes to make another pass.

The cloud-top seeding delivery system was de-
veloped in Alberta in the early 1970s based on studies
of the structure and behavior of Alberta hailstorms.
The initial technique was modified twice to reflect
changes in aircraft (from a jet to piston engined air-
craft), and again due to improved understanding of the
hail formation process in the storm. The technique is
now being used on hail projects in other countries.

Because of timing and placement accuracy, this
technique has generally been the preferred seeding
method. Still, the actual seeding coverage achieved
can vary from a high of near 75 percent for some
storms to a low of near zero.

3

Figure 2. Weather Modification Program experimental areas. The circular area between Edmonton and Calgary was the prin-
cipal target zone of the Alberta Hail Project. All hailstorms that occurred in the southern half of this area were routinely seeded
for hail suppression. Storms that occurred in the northern half were used for research. The area south of Calgary was the
ground-based seeding target area.

4

Cloud-base seeding aircraft circles or flies below the cloud in the updraft

Cloud-base seeding uses flares or generators attached area feeding it and the updraft carries the seeding

to the aircraft which burn silver iodide in acetone. The material to the upper parts of the storm.

Figure 3. Side view of a hailstorm indicating the cloud seeding methods used in Alberta. For cloud-top seeding, aircraft fly near
the top of the target cloud releasing either silver iodide flares or for some research experiments, dry ice pellets. The flares burn
as they fall through the cloud releasing billions of ice nuclei in the form of smoke particles. This produces a curtain of seeding
material. In cloud-base seeding, aircraft fly just below the base of the clouds releasing silver iodide smoke that is carried into
the cloud by its updraft. The seeding material is produced by flares burning on the aircraft wings, or by smoke generators at-
tached under the aircraft. Ground-based seeding uses a network of generators and relies upon natural air currents to carry the
ice nuclei to the clouds.

Hail

Alberta hailstorms

Investigations of the storm environment have shown
that even though the various weather systems are
vastly different in size (from single clouds to large fron-
tal systems), they are intricately linked in a chain of
events that ultimately leads to hail on the ground.
Thus, in order to formulate a complete hailstorm
model, the various links needed to be isolated and
understood. This new appreciation of the effects of
large weather systems on the development of hail-
storms has resulted in improved forecasts of storms
and a potentially useful tool for the evaluation of cloud
seeding effects.

Hail occurs in central Alberta (within a 1 30 km radius
of Red Deer) on an average of 61 days during the sum-
mer. The majority of hailstorms (85 percent) occur be-
tween noon and 11:00 pm with 90 percent of

hailswaths less than 30 km in length covering less than
200 sq. km. However, hailswaths can be over 300 km
in length and 30 km in width from storms lasting for
three or four hours. Hailstorms normally develop along
the foothills during the afternoon, then move eastward
over the farmlands and can continue producing hail in-
to Saskatchewan.

Hailstorms result from energy imbalances in the at-
mosphere-imbalances that produce storms genera-
ting power equivalent to the electricity used by a city
the size of Edmonton or Calgary in a year. Large-scale
weather systems such as low pressure areas and
fronts generate up to a thousand times as much
energy as a hailstorm. These atmospheric forces are
linked to hailfall by a complex chain of processes.
Satellite imagery shows a clear tendency for convec-
tive clouds to develop within a framework of larger

5

cloud complexes which in turn are associated with
parent weather systems. These large systems in-
fluence the atmosphere on a regional scale, producing
conditions favorable for the formation of hailstorms
many hours before the actual storm develops.

Current theory suggests that these events, in con-

Precipitation Process Chain

L Synoptics Cloud Processes

Synoptic Hail at

Environment Ground

Figure 4. The “precipitation process chain” illustrating the
sequence of weather events leading to hail. The synoptic
weather determines the storm environment which in turn
determines cloud development. The cloud development
determines if or when hail growth starts, how fast it will be and
how long it will continue. These factors determine the number
and size of hailstones that fall to the ground.

junction with the influence the mountains have on the
airflow and with solar heating, focus cloud develop-
ment along the foothills. Airflow from over the moun-
tains creates a “lid” that in the early stages sup-
presses cloud development. However, at the same
time, the “lid” increases the potential instability of the
atmosphere so that when it finally erodes, intense air
currents capable of supporting large hailstones occur.
The storms then move off the foothills depositing their
hail load.

Most hailstorms in Alberta have a series of smaller
clouds upwind of the main storm. The exact location of
these clouds appears to depend upon the available
moisture and winds in the environment. These clouds
form at regular intervals in the same location relative to
the storm and their formation appears to be deter-
mined by the interaction between the storm’s down-
draft (produced by rain and hail falling out of the
cloud), its inflow (produced by warm air rising ahead of
the storm) and the environmental winds. Because
these clouds move towards and merge with the main
storm, they are called feeder clouds. The growth of
hailstones appears to start in these feeder clouds.

The growth of a hailstone

The growth of a hailstone occurs in two stages. The
beginning stage, called embryo formation, must occur

May

June

July

August

September

Figure 5. The climatological chance of hail occurring somewhere in central Alberta on any given date during the summer.

6

Figure 6. Hailstorm tracks during 1983. The arrows indicate
the center line of the storms.

in small or young clouds with weak updrafts such as
feeder clouds. The final growth to a hailstone, though,
must occur in a strong and moist updraft such as in the
main storm.

All clouds contain millions of water droplets. These
water droplets will remain unfrozen (supercooled) even
at temperature as cold as -40°C if no ice nucleating
particles are present. Most natural ice nucleating par-
ticles in clouds are dust which are effective only at
temperatures of -15°C and colder. Therefore, once
cloud tops are colder than -15°C, ice crystals
develop. These ice crystals grow by colliding with and
collecting water droplets and smaller ice crystals in the
cloud. Although water droplets also grow, ice crystals
can grow much faster in Alberta clouds. Once an ice
particle has reached a size of 1 mm, it is a potential
hail embryo.

For a hailstone to form, a hail embryo must be
caught and carried up by a strong and moist updraft
that will keep it hovering aloft long enough to collect
and freeze thousands of water droplets and ice
crystals. This happens when the feeder cloud, in which
the embryos develop, merges with the main storm.
When the hailstone grows too heavy to be held aloft by
the updraft it falls to the ground.

Figure 7. A schematic illustration of the effects of air flow
from large weather systems crossing the mountains and the
reverse air flow towards the mountains at lower levels
creating a ‘convective lid’. Daytime heating at the ground and
approaching cooler air from the Pacific erodes the lid permit-
ting storms to develop.

Hail suppression concepts

Modern hail suppression efforts do not attempt to con-
trol the storm itself, but seek to modify only the hail
growth process within it. Most hail suppression
theories involving cloud seeding are based on what is
known as the “beneficial competition” or “competing
embryo” hypothesis. This concept assumes that the
amount of supercooled water in the hail cloud is the
limiting factor on the mass of hail produced. Introduc-
ing enough artificial hail embryos can reduce the sizes
of the hailstones reaching the ground by forcing
natural hailstones to share the available liquid water
with artificial ones. Smaller hailstones melt more readi-
ly so that the total hail mass at the ground is less,
reducing damage to crops. Artificial embryos are pro-
duced by seeding clouds with materials that produce
additional ice nuclei.

This has been the rationale behind the seeding
operations and experiments in Alberta since the early
1970s. However, beneficial competition involves the
risk of increasing the natural hailfall without
ameliorating its effects if the natural embryos do not
consume all the available liquid water or if too few ar-
tificial embryos are added.

More recently, an additional hail suppression con-
cept, known as “premature rain out”, has been pro-
posed by Soviet and other scientists. This concept
rests on the fact that ice nucleating materials such as
silver iodide can initiate ice crystals at warmer

Figure 8. An aerial view of a hailstorm. Warm moist air flowing from the lower left of the photo, rises to form feeder clouds (near
the left center of the photo) where moderate updrafts support the growth of hail embryos. The feeder clouds later merge with
the main storm which has a strong updraft in which the embryos grow to large hailstones.

Figure 9(a). The largest hailstone on record in Alberta fell
near Wetaskiwin on July 6, 1975. With a diameter of 10 cm
and weighing 249 g, it fell at 150 km/h.

temperatures than normal, close to 0°C, and therefore
earlier in the lifetime of the cloud. This should start the
precipitation process in the feeder clouds earlier and
result in potential hail embryos falling out of the feeder
cloud before it merges with the main storm so that
fewer embryos are delivered to the updraft in the main
storm.

Testing the concepts

Because both concepts— the beneficial competition
and the premature rain out— involve changing the

Figure 9(b). A thin slice through the center of a hailstone
which fell in central Alberta on July 21, 1982. This hailstone
grew from a 4 mm conical ice pellet. The alternating layers of
clear and opaque ice of the hailstone indicate that this stone
either passed through a series of different growth en-
vironments or grew in conditions that fluctuated with time.
Studies of such thin slices of hailstones provide detailed infor-
mation on how hailstones grow.

number of embryos in feeder clouds, both concepts
were tested through seeding experiments.

Hailstorm seeding experiments conducted in Alberta
have involved treating a feeder cloud and then
documenting the effect of the treatment with observa-
tions made by the cloud physics research aircraft.
Once a feeder cloud became too vigorous and too

8

Hail Suppression Concepts

Time (minutes)

0

10

20

30

40

50

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Figure 10. This illustrative slice through a typical hailstorm shows the expected paths of hailstones with and without seeding.
After their beginning as embryos in the feeder cloud, the particles grow rapidly in the main updraft to fall as hailstones. Seeding
the feeder clouds to introduce large numbers of ice crystals at the same time as natural ones are forming produces many ar-
tificial hail embryos that compete for the cloud’s water supply. This competition limits the size of all embryos, resulting in
smaller hailstones with a higher path through the storm. On this higher path (labelled beneficial competition), the stones grow
slower and fall out of the storm as smaller hail. With some clouds, seeding the feeder cloud to cause ice crystals to form earlier
allows hail embryos to precipitate out as rain before they reach the main updraft (premature rain out).

much a part of the mature storm to permit the research
aircraft to penetrate it, precipitation particles from the
feeder cloud could usually be identified and tracked by
weather radar. In addition, storm chase vehicles col-
lected hailstone samples in the precipitation area. Dur-
ing the experiments carried out in 1985, observations
were also made using an armored aircraft (a T-28 from
the South Dakota School of Mines and Technology),
which was able to fly through the center of the storm.

The treatments used in the seeding experiments in-
cluded a placebo (no seed), droppable silver iodide
flares, or dry ice pellets. With the dry ice pellets, a low
and a high rate were used, so that a total of four dif-
ferent treatments were used. The treatment was usual-
ly applied at about the - 10°C level; when the scientist
onboard the research aircraft thought that there was a
possibility of success with the premature rain out con-
cept, the treatment was applied at the -5°C level.

Test clouds were required to satisfy certain criteria
to ensure that all treated feeder clouds were similar.
Ideally four feeder clouds from one storm were treated
as part of one experiment — each cloud receiving a dif-
ferent treatment— so that differences measured could
be attributed to the treatment.

A completely successful series of seeding ex-
periments (four treatments) is extremely difficult to
achieve since identification of a suitable feeder cloud,

treatment and subsequent observation of the cloud
can take as much as an hour. A storm will often
change dramatically or move out of the project area
before an experiment is completed. In addition, the
radar data, research aircraft data, weather observa-
tions and observations of rain and hail at the ground
result in several kilometres of computer tape and
millions of data values. Checks of data quality, extrac-
tion and analysis of relevant information, comparison
of the various sources of information and interpretation
of the results take months of effort for each case.

To date, six hailstorms have been analyzed. In four
of these, a placebo as well as a seeding treatment
were successfully applied. Data from six more suc-
cessful hailstorm experiments and at least 10 addi-
tional partial experiments remain to be analyzed.

The seeding concepts can also be tested theoretical-
ly by using computer models to simulate atmospheric
conditions. One such test was conducted with the
assistance of the Institute of Atmospheric Sciences at
the South Dakota School of Mines and Technology.

Experimental results

Hailstorm seeding experiments

Results from the seeding experiments indicate that the
current understanding of how hailstorms function is

9

Figure 1 1 . An artistic rendering of a hailstorm seeding experiment. If the research aircraft’s first measurements determine that
the feeder cloud meets requirements, the seeder aircraft, following, is instructed to release a seeding treatment. Subsequent
penetrations by the research aircraft document the effects of the treatment. Cloud penetrations continue until hailstones are
too large to permit further safe cloud measurements. Then the T-28 aircraft enters the main updraft region of the storm to con-
tinue measurements of the treated cloud. Following its treatment pass, the seeder aircraft retreats to a safe distance to
photograph the storm. Chase vehicles intercept the storm to measure the hail and document the effects of the treatment.
Weather radars record the storm’s evolution and intensity of precipitation.

appropriate for Alberta and for the most part correct.
Radar analysis of isolated storms indicates that 80 to
90 percent of storms that occur in Alberta are consis-
tent with the model: storms have feeder clouds associ-
ated with them in which the precipitation process is in-
itiated, and particles which form in the feeder clouds
fall to the ground as hail.

An abundance of ice crystals was observed in all
seeding experiments soon after seeding with either dry
ice or silver iodide. The number of ice crystals in the
seeded feeder clouds was very much greater than the
number in unseeded clouds at comparable stages in
the cloud life. These high numbers of ice crystals were
intially observed in a very small part of the cloud, but
with time they spread and grew— some of them be-
coming potential hail embryos within 10 minutes.

The number of potential hail embryos in the seeded
feeder clouds was consistently about 10 times greater
than the number in the unseeded clouds (beneficial
competition concept). When seeded feeder clouds

were sufficiently distant from the main storm, seeding
encouraged the early formation of precipitable par-
ticles (premature rain out concept).

For safety reasons, the research aircraft cannot
follow the treated feeder cloud once hail embryos have
grown into centimetre-sized hailstones. Detailed
analyses of the trajectories of radar reflectivity patterns
(which are related to the particles growing within the
feeder clouds) have led to the conclusion that such
feeder clouds contribute particles which precipitate in
the hailswath. Current radar analysis techniques are
not sufficiently sensitive, however, to determine what
effect additional hailstone embryos have upon the size
and number of hailstones within the storm.

This problem was recognized after the third year of
the program, and thus the armored aircraft of the
South Dakota School of Mines and Technology was
brought to Alberta in 1985. However, 1985 proved to
be a year of low hailfall, and this aircraft successfully
participated in only one hailstorm seeding experiment

10

Figure 12. Sample results from testing the hail suppression concepts. Panel 1 illustrates the concept of hail formation. Panel 2
shows a comparison between the number of ice crystals in a seeded and unseeded feeder cloud as a function of time after
treatment. Panel 3 indicates the spreading of ice crystals in a feeder cloud as determined from a sequence of aircraft passes
through the cloud. Panel 4 shows the recorded images when ice particles intercept the laser beams onboard the research air-
craft. Panel 5(a) (beneficial competition) illustrates the difference in numbers of hail embryos in seeded and unseeded feeder
clouds. Panel 5(b) (premature rain out) shows the amount of computer-simulated precipitation from seeded and unseeded
feeder clouds. Premature rain out is indicated in the seeded cloud. Panels 6(a) and 6(b) remain to be confirmed. The bars at the
bottom of the panels indicate the degree to which that step of the suppression concept has been verified.

11

on July 11— an extremely severe storm. Analyses in-
dicate that this storm was not sufficiently well seeded
using current techniques. This was due in part to the
fact that the feeder clouds for this severe storm grew
right on the edge of the storm, allowing insufficient
time for artificially produced embryos to prematurely
rain out. In addition, there were exceptionally strong
updrafts in this storm which required more seeding
material than could be applied in the experiment.
These very intense storms appear to constitute less
than 20 percent of the damaging hailstorms that occur
in central Alberta.

Observations of hail collected at the ground from
one operationally seeded storm suggest that the
number of hailstones on the ground was increased
through cloud seeding. Not enough data were ob-
tained to determine if the size of the hailstones was re-
duced to any significant degree.

Computer calculations

The Institute of Atmospheric Sciences at the South
Dakota School of Mines and Technology used a com-
puter model to simulate the seeding experiments that
were carried out on a storm that occurred in Alberta on
July 26, 1983. The calculations show that the feeder
clouds were a source (although not the only one) of
hailstone embryos; that seeding produced many ice
crystals which spread through the cloud and grew with
time; and that the seeded feeder cloud precipitated
earlier than the unseeded feeder cloud. Furthermore,
a temporary competition for the liquid water was seen.
This suggests that seeding must be continuous, as it is
in operational cloud seeding, to maintain this competi-
tion.

The computer simulations suggest that seeding one
feeder cloud increased surface amounts of rain
modestly (approximately seven percent) and de-
creased hail amounts somewhat (approximately nine
percent). The computations verify that seeding re-
sulted in the premature initiation of precipitation in this
storm.

The results of these numerical tests cannot currently
provide definitive answers regarding the effects of
seeding because numerical models cannot yet simu-
late all the complexities of nature, but they do give in-
dications of what seems reasonable to expect and
what to look for in observations of the actual storms.

Economic assessment of hail
suppression techniques

The experiments discussed in previous sections pro-
vide encouraging indications that cloud seeding can
affect the amount of hail that falls to the ground, but
the question of what economic benefit cloud seeding
can provide can only be addressed in a properly de-
signed statistical experiment.

One type of cost-benefit assessment involves the
use of crop insurance data. A loss-to-risk ratio is often
used to assess hail damage. This is the ratio of crop
loss caused by the hailstorm to the risk or insured
value of the crop, and is usually expressed as a per-
centage. Many evaluations of hail suppression pro-
grams have used the loss-to-risk ratio, usually with

mixed results, except in the Soviet Union and France
where significant reductions in hail are claimed.

Alberta farmers have purchased insurance against
crop losses since the first settlers began farming
operations in the latter half of the nineteenth century.
The most consistent records in the province are those
of the Alberta Hail and Crop Insurance Corporation
and its predecessor, the Alberta Hail Insurance Board,
spanning the period from 1938 to the present. These
records represent the bulk of insurance policies pur-
chased by farmers within the province.

Recent analyses of crop damage data from the
Alberta Hail and Crop Insurance Corporation suggest
a decrease in the loss-to-risk ratio of the order of 20
percent could be attributed to cloud seeding if no
significant changes in weather patterns have occurred
in central Alberta in the past 50 years and if no other
factors have contributed to the observed decrease.

However, hail insurance data do not necessarily pro-
vide an accurate measurement of hailfall over an area
because factors such as wind, crop type and growth
stage determine how much damage will be caused by
a given hailstorm. In addition, trends due to climate
change, cloud seeding and changes in insurance prac-
tices are difficult to isolate. For this reason, a cost-
benefit analysis of hail suppression should not rely
primarily on insurance data.

In preparation for a future definitive cost-benefit
analysis, some effort has been devoted to determining
optimum measurements of hailfall. Results obtained to
date suggest that time-resolved hailstone samples and
some radar parameters provide measurements of hail-
fall that are accurate enough to produce definitive
results from a statistical experiment of relatively short
duration (five to ten years). Factors that influence hail,
such as particular weather patterns and storm environ-
ments, have been identified and can be used in the
future to separate effects due to changes in climate
from those due to cloud seeding. Techniques have
been developed to quantify the extent to which a storm
is successfully seeded, so that an improper application
of seeding material can be taken into account.

The Economic Services Division, Alberta
Agriculture, has analyzed the effect of hail and drought
on major crops in Alberta. In one analysis, the
estimated average hail damage in 1985 dollars for the
1980 to 1985 period is 146 million annually. All
analyses also show “that a loss recovery percentage
of under 10 percent is sufficient to break even on hail
suppression system costs’’. In particular, one analysis
indicates that “benefits equivalent to 6.5 percent of
premiums and administration costs are sufficient to
pay for the total system cost”. If secondary benefits
and total crop value are considered, the loss recovery
can decrease to less than three percent.

Summary and conclusions

The emphasis of the weather modification program
over the past five years has been to better understand
the hail formation process and the effect cloud seeding
has upon it. A storm model was formulated which des-
cribes the current state of knowledge. This model has
been shown to be appropriate for at least 80 percent of

12

the hailstorms that occur in Alberta. Seeding concepts
have been tested with seeding experiments and with
computer calculations.

Results from the six hailstorm seeding experiments
indicate that cloud seeding can increase the number of
potential hail embryos produced by the feeder clouds
or, in some storms, can cause potential hail embryos
to precipitate out of the feeder cloud prematurely.
Because of limitations in measuring and observing
facilities, it has not yet been demonstrated that more
hail embryos lead to smaller hail on the ground, nor
that premature rain out of embryos results in fewer
hailstones on the ground.

Computer calculations for one hailstorm have con-
firmed that premature rain out from the feeder cloud
occurred. The calculations suggest that this resulted in
a moderate decrease in the amount of hail (approx-
imately nine percent) and a moderate increase in the
amount of rain (approximately seven percent).

Statistical analysis of crop damage data from the
Alberta Hail and Crop Insurance Corporation suggests
that a decrease in the loss-to-risk ratio of the order of
20 percent could be attributed to cloud seeding if
changes in weather patterns and insurance practices
have had no significant effect on this ratio.

Statistical analysis of hailstones from untreated
storms and from one operationally seeded storm
shows that cloud seeding increased the number of
hailstones on the ground. However, not enough data

Rain

The potential of rainfall
augmentation

Rainfall over much of southern Alberta is barely ade-
quate in an average year, yet natural variations are
such that the rain received in the growing season can
be as much as 30 percent below normal once every
five years. Even worse, much of this area can expect
precipitation during July to be 60 percent below
average once every five years.

Even when precipitation is averaged over five sum-
mers, large departures from the 30-year norm are fre-
quently observed. Summers of the past decade have
been characterized by below-average rainfall along the
Bow River with above-average rainfall in the Drayton
Valley area and the regions to the north and west of
Edmonton. Southern Alberta has seen wide changes
from very dry (as much as 40 percent below average in
the late 1960s and early 1970s), to very wet (up to dou-
ble the average rainfall in the mid-1970s) and back to
dry again in the early 1980s. Departures of 30 or 40
percent above or below normal are common. During
1984 and 1985, the rainfall was very much below nor-
mal.

Weather modification is one means of increasing
rainfall. Cloud seeding has been seen as a means of
increasing the water supply in many areas of the world.
Particularly well known is a project in Israel which used
silver iodide to seed winter convective clouds. A statis-
tical analysis showed that seasonal precipitation was
increased by 15 percent.

were obtained to determine if the size of the hailstones
decreased significantly.

Aside from these results on the feasibility of sup-
pressing hail through cloud seeding, significant pro-
gress has been made in other areas. The time and
location of storm development and the size of hail can
be more accurately forecast. A foundation of
knowledge has been built up whereby it is now possi-
ble to study the effect of cloud seeding on the storm
environment and hence on the development of new
clouds and storms. A technique has been devised to
separate effects of the storm environment and seeding
coverage from effects due to cloud seeding on hailfall
and a method of quantifying seeding effectiveness has
been developed.

These results give reason to be optimistic about the
possibility of suppressing hail to some extent in the
most common types of storms and of detecting and
economically assessing the suppression effect. The
potential benefit to the agricultural community of cloud
seeding for hail suppression is great and the cost of
seeding is small in comparison. Although the final step
in the hail suppression process (cloud seeding to re-
duce hail damage) has not been validated, encourag-
ing indications that cloud seeding can affect hailfall on
the ground have been found. No evidence has been
found that cloud seeding cannot succeed. Thus opera-
tional cloud seeding for hail suppression appears to be
a positive tool for agriculture.

In Canada, the Atmospheric Environment Service
carried out seeding experiments on cumulus clouds in
Yellowknife and Thunder Bay. Clouds in both areas
showed increases in ice concentration with seeding
and, in Yellowknife, 40 percent of the clouds produced
rain.

Cumulus seeding experiments in Alberta in 1978
and 1979 produced dramatic radar evidence of rain
augmentation through cloud seeding. These ex-
periments and the encouraging results reported from
elsewhere, led to the initiation of a series of rain
seeding experiments in Alberta.

Rainfall augmentation concepts

Modern rain augmentation projects have been based
on the belief that convective clouds are inefficient rain
producers because of a lack of natural ice nuclei. It is
believed that seeding convective clouds with an ice
nucleating material will convert more of the cloud
water to ice. Ice particles will grow faster than water
droplets and so will become heavy enough to fall out of
the cloud faster than water droplets. The ice particles
melt as they fall to the ground and arrive as rain.

Testing the rainfall augmentation concept through
cumulus cloud seeding experiments has been a
secondary objective of the weather modification pro-
gram. These experiments were designed to document
the processes of rain development in natural and seed-
ed cumulus clouds, to determine the range of condi-

13

Figure 13. Rainfall in Alberta varies considerably from region
to region and from one period to another. This map shows the
1974 to 1985 summer rainfall (July + August) as a departure
from the 30-year normal from 1941 to 1970. The shaded
areas show regions with more than 20% departure from nor-
mal. No clear pattern consistent with the cloud seeding areas
is evident.

tions under which clouds can be made to rain through
cloud seeding, and to determine the most effective
treatment.

Rainfall in Alberta is produced by widespread
weather systems, thunderstorms and cumulus shower
clouds. This rain is usually melted snow or graupel that
grew in the cloud from tiny ice crystals. Since cumulus
clouds with tops warmer than - 15°C do not tend to
rain naturally, such clouds were chosen for experimen-
tation to simplify evaluation of the effect of seeding.

Seeding experiments were carried out whenever iso-
lated cumulus clouds or cumulus clouds embedded in
a stratus cloud deck occurred in the project area and
equipment was not required for hailstorm seeding. The
experiments involved choosing a test cloud according
to characteristics measured by the research aircraft.
An appropriate treatment of either silver iodide or dry
ice pellets, or a placebo (no treatment), according to a
random sequence, was applied to the test cloud. The
treatment effects were observed by repeated flight
through the treated cloud by the research aircraft.
Radar observations were also made to monitor precipi-
tation development in the cloud.

Results of the seeding experiments

During the period 1982 to 1985, 98 experiment clouds
were selected. Ten clouds had to be rejected during
analysis due to missing information. Out of the 88
clouds analyzed, 57 had been treated with a seeding
agent and 31 were treated with a placebo (control
clouds).

Observations with the research aircraft show that
the class of cumuius clouds selected for the ex-
periments do not naturally produce high concentra-
tions of ice crystals. Seeding these clouds with either
silver iodide or dry ice is effective in producing high ice
crystal concentrations and these ice crystals spread

through the cloud and grow with time.

None of the test clouds that lasted less than 20
minutes developed precipitation. Twelve of the control
clouds lasted for 20 minutes or longer, but none of
these developed precipitation. Thirty-six seeded test
clouds lasted for 20 minutes or longer after the
seeding agent was applied and developed precipita-
tion (i.e. 63 percent of the seeded clouds were ob-
served to have rain fall out of the cloud at cloud base).
Twenty of the 36 clouds were suitably located to deter-
mine if rain fell to the ground. It is estimated that 35
percent of the seeded clouds that lasted for 20 minutes
or longer produced rain on the ground or 22 percent of
all seeded clouds produced rain on the ground. All the
test clouds that produced rain on the ground were
seeded with silver iodide.

Rainfall estimates calculated from the radar data in-
dicate that the test clouds produced up to 1 6 000 cubic
metres of rainwater when seeded and covered areas
up to 140 square kilometers which averages to 0.1 mm
of rain. In one case, 0.9 mm of rain was measured at
the ground.

Cloud climatology studies

To enable estimates of the potential impact of the
results of the seeding experiments on regional rainfall,
a cloud frequency study was undertaken. This study
showed that on average, during the summer months, 7
to 1 1 percent of the sky in southern Alberta is covered
with the type of clouds investigated in the seeding ex-
periments. The study also indicated that clouds of the
type used in the experiments are just as prevalent dur-
ing a drought as they are during a non-drought period.
The climatology studies suggest that an additional 10
mm of rain could be realized from seeding isolated
cumulus clouds during the summer. With larger cloud
systems this could be increased even more.

Alternative seeding techniques

Cloud-base seeding

An exploratory study was undertaken during the 1985
field season to test the cloud-base seeding technique
for accuracy of delivery. Results from four experiments
showed that silver iodide delivered at cloud base was
transported into the cloud. Distinct seeding effects
were observed similar to those seen with cloud-top
seeding. Therefore, although the timing may be more
difficult to control, it seems that seeding at cloud base
is an alternative to seeding within, or above, a cloud
near its top.

Ground-based seeding

An alternate method to both cloud-top and cloud-base
seeding with aircraft is seeding from the ground using
some form of silver iodide generator. Such devices
have the advantage of simplicity, ease of access, no
aircraft operating costs and manual operation.

A project to evaluate the efficiency of seeding sum-
mer clouds using ground-based silver iodide genera-
tors was also conducted as part of the weather
modification program. This technique has been used
by I.P. Krick and Associates in Alberta and elsewhere

14

for more than 30 years. The test system involved a net-
work of the coke-fueled and arc generators installed at
60 to 70 sites in southern Alberta during the summers
1981 through 1985. The coke generators burn pellets
of coke that have been soaked in a solution which con-
tains silver iodide, while the arc generators burn pure

silver iodide by means of an electric arc. The opera-
tional objective was to increase rainfall in a 15 000
square-kilometre target area south of Calgary.

Mapping and plume-tracking flights conducted in
conjunction with the I.P. Krick ground-generator
operations in southern Alberta showed that the

Sequence of Events Leading to Increased Rainfall

Bar scales indicate a percentage confirmation of the steps

Figure 14. Sample results from rainmaking experiments. The panels indicate steps in the rain augmentation concept and the
bars show the degree to which each step has been verified. Panel 1 illustrates the concept of rain formation in an Alberta
cumulus cloud. Ice crystals form on ice nuclei in the cloud’s updraft and grow as the cloud provides moisture. When the updraft
dies, the particles fall, and melt to form rain. Panel 2 shows the number of ice crystals in a seeded and a natural cloud as a
function of time. Panel 3 indicates the spreading of ice crystals in the cloud. Panel 4 shows the images recorded when ice par-
ticles and water drops intercept the laser beams onboard the research aircraft during several passes after seeding. Panel 5 il-
lustrates that for the clouds which last 20 minutes or longer after seeding (63%), ice particles grew large enough to fall out of
the cloud. Panel 6 shows that 22% of the seeded clouds produced rain on the ground.

15

generators typically produced narrow plumes a few
hundred metres wide occupying a few percent of the
target volume. Occasionally, plumes were encoun-
tered near cloud base.

Laboratory tests of the generators by Colorado State
University showed they did produce effective ice form-
ing nuclei albeit at lower rates than other systems.

An independent statistical evaluation of the effect of
ground-generator cloud seeding in southern Alberta
was carried out by the University of Alberta. This study
concluded from three different analyses that no evi-
dence of a change in rainfall greater than 12 percent
could be found.

Summary and conclusions

The cumulus seeding experiments have shown con-
clusively that some cumulus clouds that would not rain
naturally can be made to rain by seeding with an ice
nucleating material. Of the clouds seeded in the
course of these experiments, 22 percent produced rain
on the ground and rainfall was observed at cloud base
in 63 percent. While half the test clouds were treated
with dry ice, all of the clouds that produced rain on the
ground were seeded with silver iodide.

Snow

Potential of snowpack augmentation

Inadequate rainfall during the growing season can be
alleviated in some areas through irrigation. Much of
the water currently being used to irrigate crops in
southern Alberta comes from spring runoff, and it is
estimated that a 10 percent increase in the average
cumulative mountain snowpack would result in
250 000 acre-feet of increased runoff in the Oldman
River basin alone, adding greatly to irrigation and
other water supplies. Municipalities would also benefit
from additional snowpack.

Research and development into cloud seeding for
snowpack augmentation is underway in the western
United States to help alleviate potential water short-
ages. Projects are currently being conducted in
California, Colorado, Montana, Nevada and Utah by
various universities, state and federal agencies, in-
cluding the U.S. Department of the Interior and the Na-
tional Oceanic and Atmospheric Administration
(NOAA). Cloud seeding in the mountains has been
identified recently by the United States Secretary of
the Interior as “the most cost effective and promising
means of meeting the water needs of the Colorado
River basin.’’

U.S. results indicate that cloud seeding may in-
crease the mountain snowpack by about 15 percent. In
Colorado, it has been estimated that such increases
could augment streamflows by 10 percent.

Snow augmentation concept

Cloud seeding for snow augmentation is also based
upon the fact that the atmosphere is not always
naturally “efficient” in producing precipitation. In a

Seeding at cloud base with aircraft seems to be a
viable alternative to seeding at cloud top. No conclu-
sions can yet be drawn about the effectiveness of
seeding from the ground with ground-based
generators.

The experiments conducted to date have dealt with
a very specific type of cloud which is equally prevalent
in times of drought and of normal rainfall.

These results indicate that cloud seeding for rain
augmentation should be considered as a tool for water
management. However, before the economic potential
of this tool can be assessed, further experiments are
required to optimize the seeding techniques and to
assess the potential of increasing rain from other types
of summer clouds. Before appropriate clouds can be
seeded routinely, it must be determined if there are
any kinds of clouds for which seeding would decrease
rainfall. If such clouds exist, the seeding technique
must be able to exclude them in routine seeding opera-
tions.

Finally, a properly designed statistical experiment
will be required to determine economic benefits. Such
an experiment should include considerations of down-
wind effects, that is, the possibility that cloud seeding
in one area robs some area downwind of its rain.

moderate westerly airflow over a mountain ridge, with
clouds one km deep and extending one hundred km
along the mountains, about 5000 cubic metres (one
million gallons) of water, pass over the ridge each
minute. Because of a scarcity of ice nucleating par-
ticles, supercooled water droplets in these clouds
often do not freeze even at temperatures much colder
than zero. In the absence of ice nuclei, the cloud
droplets evaporate in the descending air on the
leeward side of the mountain barrier.

In a weather modification program, ice nucleating
agents (either silver iodide or dry ice pellets) are
delivered to the cloud to initiate snow crystal growth.
The seeding principles for converting cloud water to
snow are very similar to those for rain augmentation.

Alberta investigations

A limited investigation of the feasibility of increasing
snowfall over the southern part of the Alberta Rocky
Mountains was carried out during the past five years.

A preliminary assessment of the snow climate and
clouds of the southern Canadian Rockies was recently
completed which investigated the snow climatology of
the region and measured the properties of winter
clouds over the mountains. The snow climatology
showed there are different snowfall patterns on each
side of the continental divide. The spring contribution
to the total snowpack was noted. The climate studies
indicated that meteorological conditions within the
region were appropriate for cloud seeding technology.

Measurements by the research aircraft during four
two-week field programs from 1982 to 1984 showed
evidence of liquid water in the clouds and an increase

16

in liquid water near barrier peaks. Estimates for three
selected cases indicated less than one percent to 16
percent of the moisture was converted to snow. These
results suggest that the precipitation process could be
made more efficient if artificial ice nuclei were added.

Atmospheric research to study weather modification
requires a broad base of expertise in such disciplines
as meteorology, statistics, computing and electronics
as well as a broad range of supporting technologies.
The Alberta Research Council has developed a first-
class team of experts and new technologies that can
be applied to other types of research and problem solv-
ing with a positive impact on the province.

A major component of the program was the cloud
physics research aircraft developed jointly with
INTERA Technologies through funding from Alberta
Agriculture and the Alberta Research Council. This air-
craft’s instrumentation can measure many cloud-
related and atmospheric parameters such as tempera-
ture, humidity, pressure, cloud droplets, raindrops and
winds. The data is stored, processed and displayed in
the aircraft with the aid of a computer data acquisition
system.

The research methodology and cloud seeding tech-
nology has assisted INTERA in the international
marketplace. They are actively marketing the research
aircraft facility as well as the weather modification
technology they have acquired by participating in the
Alberta project. Other countries have expressed an in-
terest in involving INTERA Technologies and the
research aircraft in their weather modification pro-
grams to the extent they now have a three-year,
$10 million weather modification contract with the
Greek government. This successful marketing of
Alberta expertise and technology would not have been
possible without the weather modification program in
Alberta.

The meteorological expertise associated with the
weather modification program has been contracted to
Alberta Environment to support studies of air quality in
Fort McMurray and Calgary. The research in Fort
McMurray involved using the research aircraft to track
effluent plumes from oil sands processing plants.

The Alberta Research Council is currently finalizing
arrangements with the University of Dayton, Ohio, to
supply aircraft data collected during the program to be
used in updating icing standards for commercial air-
craft.

The computing expertise used in weather modifica-
tion research has been very useful in applying many of
the technologies to unrelated projects. All aspects of
the research and operations now rely on computing to
store and analyze data from the research aircraft, the
weather radar systems, the weather information from

Summary and conclusions

Limited observations from snow clouds over the Rocky
Mountains in southern Alberta show that potential ex-
ists to increase mountain snowpack. Therefore, a pro-
ject called SNOWATER has been proposed to adapt
snow augmentation technology for use in Alberta.

program

the Atmospheric Environment Service and even the
data obtained from telephone surveys of farmers.
Some of the programs used to support the graphic
representation of the data will soon be used by Alberta
industry. Other programs used to access and store the
meteorological data from the Atmospheric Environ-
ment Service have been used by Syncrude in their en-
vironmental work.

Statistical expertise used in weather modification
evaluations has also been applied to crop yield model-
ling for Alberta Energy and Natural Resources and in
oil production studies for AOSTRA.

Research on hailstorms has improved the ability to
forecast convective weather. A computer program that
produces weather forecasts in a manner similar to the
human meteorologist (using artificial intelligence
techniques) has been developed and may soon be in
the private sector. In addition to aiding weather
modification research and cloud seeding operations,
improved forecasts of hailstorms provide more ac-
curate and timely warnings for the public so that
precautions can be taken such as diverting flights, put-
ting aircraft and vehicles under protective covering,
drawing blinds to protect against broken glass and
seeking cover from large hail which can inflict severe
injury.

Weather radar is needed to detect and track storms
for weather modification research. Radar can also be
used to estimate rainfall over broad areas, which is
useful in predicting stream flow. A facility has been ad-
ded to the weather radar system in Red Deer that has
the capability of transmitting measurements to the
Alberta River Forecast Centre in Edmonton as rainfall
occurs. In addition, computing technology has been
adapted so that radar information from the Atmos-
pheric Environment Service’s weather radars in
Vulcan and Stony Plain can be collected and transmit-
ted to the River Forecast Centre.

The radar data collected as part of the weather
modification program have been analyzed for Alberta
Disaster Services to assist in assessment of severe
weather events and applied to the development of
forecasts of damaging lightning for the Canadian Elec-
trical Association. Radar data are also being used by
Alberta Government Telephones to study signal fading
during heavy rainfall.

These contracts indicate that other Alberta agencies
are developing a reliance on the weather radar system
that supports weather modification research.

Technological benefits of the

Recommendations

17

Hail

1. Major progress has been made in understanding
the hailstorm and the formation of hail, but several
basic questions remain:

• how does seeding affect the energy and
organization of a storm,

• are feeder clouds the prime source of hail em-
bryos,

• can the available liquid water in the feeder clouds
be depleted by cloud seeding,

• what happens to the embryos once in the main
storm, and

• can the inflow be adequately seeded?

It is recommended that a hail suppression
research program be conducted to address these
important issues. In particular, seeding ex-
periments should be conducted in which all (or
many) of the feeder clouds existing at any given
time are treated in anticipation that such action
would result in a radar-detectable effect on the main
storm. Also, research efforts should be directed to
improving application of the special capabilities of
the Alberta Research Council’s radar, to understan-
ding the role of the storm environment, to observing
the inflow region of storms, and to applying three-
dimensional computer models of hailstorms.

2. It is recommended that support be given to
hosting and funding Alberta’s participation in an
international project called Hailswath II. This pro-
ject would see participants from the United States
and Europe gather in Alberta in 1988, along with
their sophisticated equipment, to carry out an inten-
sive study of the hailstorm. This would mean that
additional weather radars, research aircraft and
other types of observational systems would be
directed toward the understanding of Alberta
hailstorms. Scientists from the participating coun-
tries would then analyze the data to answer ques-
tions about Alberta storms.

3. Computer calculations to date clearly suggest that
the outcome of cloud seeding depends upon the
seeding material used, the amount delivered and
the delivery method. In order to optimize cloud
seeding, it may be necessary to tailor the seeding
technique to the specific storm to be treated. In fact,
the very intense storms that do not presently appear
to respond to current seeding techniques, may be
treatable in some quite different fashion. It is
recommended that investigations of and ex-
perimentation with different ways of seeding
with aircraft and from the ground be carried out.
It is felt that the efficiency of seeding delivery can be
improved, enhancing the possibility of substantial
economic benefits from hail suppression.

4. Weather modification to suppress hail has a very
high potential economic benefit to the province.
There is increased optimism about weather modifi-
cation because of the research successes of the last
five years. It is recommended that an operational

cloud seeding experiment be established. The
goal of this program would be to evaluate the
costs and benefits of operational cloud seeding.

To provide a comprehensive statistical evaluation,
this program should be carried out over a single
area as a 50/50 randomized test. With current
knowledge and techniques, a conclusive evaluation
would take five to 10 years. Once a decision is made
on the type of confirmatory experiment that will be
permitted a comprehensive design would take
about one year.

5. It is recommended that any operational cloud
seeding should be carried out by the private sec-
tor. However, a strong link should be maintained
between the research and operational components
of any future weather modification program. This
link should include the sharing of resources to make
the program as cost-effective as possible.

6. As part of a definitive cost-benefit assessment of
cloud seeding for hail suppression, accurate
measurements of hailfall are required. It is recom-
mended that time-resolved hail observations at
the ground be emphasized, in any future pro-
gram, since this type of observation promises to be
an effective evaluation tool.

7. Predicting the amount of hail produced by a storm
would enhance the ability to detect the effects of
cloud seeding. This requires a good understanding
of the interactions of the storm with its environment.
It is recommended that studies of the storm en-
vironment and storm forecasts continue. The
continuation of such studies are important to deter-
mine if seeding will produce any adverse effects
some distance away; whether seeding will alter the
region’s climate; whether “early seeding” can sup-
press storm development and produce widespread
showers instead of an intense hailstorm.

Rain

1 . It is recommended that seeding experiments to
increase the rainfall from summer clouds in cen-
tral and southern Alberta be continued with a
greater emphasis on larger clouds and on op-
timizing seeding techniques. Larger clouds will
entail investigating the possibility of cloud seeding
increasing the rainfall from clouds that rain natural-

iy-

2. In preparation for an area-wide experiment, it is
recommended that the problem of overseeding
some clouds be investigated. The maximum as
well as the minimum amount of seeding material
that can produce a beneficial effect on rainfall from
a particular cloud must be known if all clouds in an
area are to be seeded with an average rate.

3. It is recommended that a statistical experiment
be designed to assess the cost-benefit ratio of
operational cloud seeding for rainfall augmenta-
tion. Elements of the design would include the
specific measurements of rainfall and other factors

18

that are to be used, the seeding effectiveness and
how it is to be quantified, bias in the selection of
seeding events and how it is to be handled.

Snow

It is recommended that snowfall augmentation
research be initiated and that a project called
SNOWATER, proposed as a mechanism for the

adaptation of snowfall augmentation technology
for Alberta, should be funded.

Technology development

It is recommended that the application of expertise
and technology developed for weather modification
research to other problems be continued and that
the transfer of appropriate technology to the
private sector also be continued.

Conclusion

The highlight of the recent program has been the suc-
cess of the rain augmentation project. This aspect of
the weather modification technology has been solidly
demonstrated on a limited scale. The hail problem is
the most complex and difficult to solve, but substantial
progress has been made and hail suppression ap-
pears to be feasible at least for some types of storms.
The potential to increase snowfall through cloud

seeding has been demonstrated and the technology is
promising. The potential economic gains from cloud
seeding are substantial. Results of the last five-year
program lead to optimism about the use of cloud
seeding to benefit the province’s economy and
technological capabilities. It is recommended that
weather modification receive continued support.